This paper is to study the influence of reservoir water level changes on the stability and deformation of steep slopes. Taking the left-bank slope of Jinping hydropower station as an example, based on the saturated seepage theories, the seepage is calculated under the changes of water level. In addition, the factor of safety of the slope is worked out with the Vector-Sum FoS Method during the seepage. New findings are as follows:when the reservoir water level has a rate of 1m/d from the elevation1880m to 800m, the safety factor of dam on the left bank of the head slip failure mode is reduced from 1.214 to 1.147, and the safety factor approaches a determined value as the water level tends to be stable. When the drawdown speed reaches 2m/d, the safety factor decreases more quickly; The maximum displacement of the slope is located at the shear outlet position of the potential sliding surface of the slip failure mode at the dam.